Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The magnitude of electric field to be applied to transfer an appropriate amount of toner depends on a variety of factors. For example, it is known to adjust transfer bias based on which side of the receiver is being printed, the width of the receiver, or environmental factors such as the temperature. One such factor is resistance of a transfer belt. U.S. Pat. No. 6,477,339 describes measuring the resistance of a transfer belt by applying a constant current and measuring the voltage required to sustain that current. However, this method requires mechanical contact with the transfer belt. Contaminants on receivers can transfer to the belt or roller, or vice-versa. Other types of contact current measurements can be done and have the same drawbacks. Non-contact AC measurements have also been attempted, but such measurements require maintaining a tight tolerance on the air gap between the measurement electrodes and the surface of the receiver. This is difficult, and can require adjustment of the electrode position every time the receiver thickness changes, increasing the time required to set up a job and decreasing printer productivity.
Another factor that affects transfer performance is receiver capacitance. This changes with each receiver due to mechanical variations in the shape and thickness of the receiver, and can change with environmental conditions (e.g., as the moisture content of a paper receiver changes). Various schemes have been tried, including on-line mechanical measurements of actual paper thickness and AC bridge measurements for measuring the dielectric properties of the receiver. However, mechanical measurements or any contact measurements subject the measurement equipment to wear and possible damage. Contact measurements also limit the types and sizes of receiver that can be used. Moreover, mechanical thickness measurements cannot determine moisture content of a receiver or the corresponding electrical thickness.
There is a continuing need, therefore, for an improved printing system that measures receiver capacitance and optionally adjusts transfer characteristics based on that measurement.